This invention relates to a process for the production of optically active amino acids from nitrile compounds, particularly from racemic .alpha.-aminonitrile compounds or derivatives thereof by making use of microorganisms.
Optically active amino acids such as L-.alpha.-amino acids and D-.alpha.-amino acids are important chemical substances useful in a variety of fields such as foods, feeds, pharmaceuticals and cosmetics.
Conventionally, the fermentation, synthetic, enzymatic and extraction processes have been known to produce these optically active amino acids. Among these processes, the synthetic process comprises synthesizing first DL-.alpha.-amino acids in accordance with the Strecker process or a modification process thereof and then subjecting the amino acids to optical resolution to produce L-.alpha.-amino acids. In the optical resolution, the enzymatic process using aminoacylase or similar processes have been employed (Kagaku Zokan 97, Progress of Asymmetric Synthesis and Optical Resolution, Kagakudojin, 1982, p. 175).
However, the production of L-.alpha.-amino acids according to the above-described synthetic process requires increased costs in the stage of optical resolution. For such economical reasons, the process has recently suffered gradual share reduction in the production of amino acids.
An attempt has also been made reportedly to produce .alpha.-amino acids from DL-.alpha.-aminonitriles, which are intermediates in the synthesis of .alpha.-amino acids according to the aforementioned Strecker process and represented by the following general formula: ##STR1## wherein R is an alkyl group, phenyl group or the like, by making use of microorganisms [Y. Fukuda et al., J. Ferment. Technol. 49, 1101 (1971)].
According to this report, however, DL-.alpha.-aminopropionitrile and DL-.alpha.-aminoisovaleronitrile are hydrolyzed in the presence of microorganisms belonging to Corynebacterium sp. to produce respectively DL-alanine and DL-valine, and therefore L-.alpha.-amino acids can not be obtained directly. It has also been reported that amino acids are produced by the hydrolysis of DL-.alpha.-aminonitriles in the presence of Brevibacterium sp. R312 [J. C. Jallageas et al., Adv. Biochem. Engineer. 14, 1 (1980)]. According to the report, only DL-.alpha.-amino acids are obtainable from DL-.alpha.-aminonitriles. In this connection, although the above report discloses a simultaneous production of L-.alpha.-amino acids and D-.alpha.-amino acid amides from DL-.alpha.-aminonitriles by the use of Brevibacterium sp. A4, a mutant obtained from Brevibacterium sp. R312, no description has been found with regard to the direct and exclusive production of L-.alpha.-amino acids from DL-.alpha.-aminonitriles. Further, this process necessarily involves such complex operation of separating an L-.alpha.-amino acid from a D-.alpha.-amino acid amide and hence is unfavored.
It has also been disclosed to produce amino acids by irradiating light to the reaction system before completion of the microbiological hydration of DL-.alpha.-aminonitrile compounds (Japanese Patent Laid-Open No. 162191/1986). However, only DL-.alpha.-amino acids are obtainable by this process. Thus, no report has so far been made to produce L-.alpha.-amino acids directly from racemic .alpha.-aminonitrile compounds.
As processes for producing D-.alpha.-amino acids, it has been known to produce, for example, D-alanine by reacting 5-substituted hydantoin with D-hydantoinase (Japanese Patent Publication No. 1909/1981), by reacting N-acetyl-D-alanine with D-aminoacylase (Japanese Patent Publication No. 36035/1978), or by reacting an amino acid amide with a microorganism having the activity of D-amino acid amide hydrolysis (Japanese Patent Laid-Open No. 274690/1986). These processes however need to use the expensive starting materials and to undergo several steps of reactions, so that the processes are rendered complicated disadvantageously.
In view of the aforementioned existing state of art, the present inventors have made intensive investigations and finally found that L-.alpha.-amino acids or D-.alpha.-amino acids are selectively produced when microorganisms belonging to Rhodococcus sp., Arthrobacter sp., Mycobacterium sp., Nocardiopsis sp. and Bacillus sp. and having the abilities of nitrile hydrolysis are caused to act on .alpha.-alkylideneaminonitrile compounds. The present invention has been completed on the basis of this finding and makes it the object to provide a process for the direct production of optically active amino acids such as L-.alpha.-amino acids and D-.alpha.-amino acids from .alpha.-aminonitrile compounds or derivatives thereof by making use of microorganisms selected from specific genera and having nitrile-hydrolyzing abilities.